The present invention relates to an arrangement in combustion chambers for burning solid fuel, such as firewood, wood chips, pellets, coal and sundry waste, by recirculation combustion, and including a combustion chamber with its lower section provided with fire bars, a convection section, a flue gas outlet and a fan for optionally supplying an air or an air and flue gas mixture.
For both domestic boilers and large boilers or furnaces, the most pure flue gases possible and as small amounts of excess air as possible are sought for during combustion. Since extra purification equipment is expensive, the flue gases from the boiler should furthermore be so pure that it is not necessary to use any such extra equipment. The desired slight excess of air causes a minor gas flow through the boiler, which signifies a reduction of the pressure drop in the convection section to a minimum. This advantageously affects running costs and also decreases the capacity requirement of the fan. With slight excess air there is also obtained a higher flame temperature, which is advantageous with regard to the transmission of energy in the boiler.
In the firing techniques applied up to now for firing with solid fuel, it has been found difficult to keep the amount of excess air small. According to a recently published Swedish investigation it has been found that the excess air varied between 400 and 500 percent for normal firing and loading conditions. It is obvious that in such cases the firing economy will be very poor and that the amount of pollutants in the flue gases is large. This has also been found by the investigation. Since the highest flame temperature should be about at least 1000.degree. C. in the primary zone of a flame, and rapidly taper off towards the outer portions of the flame, a very large proportion of the air will namely only pass between the flame and the walls of the combustion chamber without participating in combustion and with the result that instead it cools both the flame itself and the combustion gases.
It is known that improved combustion in firing with fuel oil as well as with solid fuel is achieved by recirculating a portion of the used gases back to the primary zone of the flame. The recirculated flue gases can either be mixed with fresh air outside the boiler or also directly inside the combustion chamber.
In a heavy reduction of the excess air, the possibility is also reduced for the formation of oxygen-rich sulphur and nitrogen compounds such as SO.sub.3 etc. There are also indications that the excess air is directly proportional to the so-called POM-formations, i.e. all the probable cancer-developing substances included in the designation "polyaromatic hydrocarbons" (POM). If the oxygen excess, i.e. the air excess, can be reduced to an absolute minimum during combustion, the POM quantities should thus also be small.
A prerequisite for reducing the soot emission in the flue gases is that the mean flame temperature is kept very high, at least 1000.degree. C., and preferably higher. If the whole of the temperature increases is to take place within a single zone, it must be rapid and require very high temperatures within the gasification zone of the fuel, which leads to the production of fly ash. This results in damaging effects both with regard to the final composition of the flue gases and with regard to the fact that fly ash is very corrosive to metals, with all the problems this implies.
Three different phases may thus be distinguished in the combustion sequence, with regard to achieving high efficiency and a high degree of purity in the emitted combustion gases at the same time. In the first of these phases the fuel will be quickly gasified at a temperature lower than the melting point of the ash, i.e. lower than 1000.degree. C., in the second the gas will be burned as rapidly and effectively as possible with low excess air and the third phase will comprise burning fuel residues and combustible gas residues, which is particularly applicable with so-called long-flame fuels.